The CD40 antigen is a 50 kDa cell surface glycoprotein which belongs to the Tumor Necrosis Factor Receptor (TNF-R) family. (Stamenkovic et al., EMBO J. 8:1403-10 (1989).) CD40 is expressed in many normal and tumor cell types, including B lymphocytes, dendritic cells, monocytes, macrophages, thymic epithelium, endothelial cells, fibroblasts, and smooth muscle cells. (Paulie S. et al., Cancer Immunol. Immunother. 20:23-8 (1985); Banchereau J. et al., Adv. Exp. Med. & Biol. 378:79-83 (1995); Alderson M. R. et al., J. of Exp. Med. 178:669-74 (1993); Ruggiero G. et al., J. of Immunol. 156:3737-46 (1996); Hollenbaugh D. et al., J. of Exp. Med. 182:33-40 (1995); Yellin M. J. et al., J. of Leukocyte Biol. 58:209-16 (1995); and Lazaar A. L. et al., J. of Immunol. 161:3120-7 (1998).) CD40 is expressed in all B-lymphomas and in 70% of all solid tumors. Although constitutively expressed, CD40 is up-regulated in antigen presenting cells by maturation signals, such as LPS, IL-1β, IFN-γ and GM-CSF.
CD40 activation plays a critical role in regulating humoral and cellular immune responses. Antigen presentation without CD40 activation can lead to tolerance, while CD40 signaling can reverse such tolerance, enhance antigen presentation by all antigen presenting cells (APCs), lead to secretion of helper cytokines and chemokines, increase co-stimulatory molecule expression and signaling, and stimulate cytolytic activity of immune cells.
CD40 plays a critical role in B cell proliferation, maturation and class switching. (Foy T. M. et al., Ann. Rev. of Immunol. 14:591-617 (1996).) Disruption of the CD40 signaling pathway leads to abnormal serum immunoglobulin isotype distribution, lack of CD4+ T cell priming, and defects in secondary humoral responses. For example, the X-linked hyper-IgM syndrome is a disease associated with a mutation in the human CD40L gene, and it is characterized by the inability of affected individuals to produce antibodies other than those of the IgM isotype, indicating that the productive interaction between CD40 and CD40L is required for an effective immune response.
CD40 engagement by CD40L leads to the association of the CD40 cytoplasmic domain with TRAFs (TNF-R associated factors). (Lee H. H. et al., Proc. Natl. Acad. Sci. USA 96:1421-6 (1999); Pullen S. S. et al., Biochemistry 37:11836-45 (1998); Grammar A. C. et al., J. of Immunol. 161:1183-93 (1998); Ishida T. K. et al., Proc. Natl. Acad. Sci. USA 93:9437-42 (1996); Pullen S. S. et al., J. of Biol. Chem. 274:14246-54 (1999)). The interaction with TRAFs can culminate in the activation of both NFκB and Jun/AP1 pathways. (Tsukamoto N. et al., Proc. Natl. Acad. Sci. USA 96:1234-9 (1999); Sutherland C. L. et al., J. of Immunol. 162:4720-30 (1999).) Depending on cell type, this signaling leads to enhanced secretion of cytokines such as IL-6 (Jeppson J. D. et al., J. of Immunol. 161:1738-42 (1998); Uejima Y. et al., Int. Arch. of Allergy & Immunol. 110:225-32, (1996), IL-8 (Gruss H. J. et al., Blood 84:2305-14 (1994); von Leoprechting A. et al., Cancer Res. 59:1287-94 (1999); Denfeld R. W. et al., Europ. J. of Immunol. 26:2329-34 (1996)), IL-12 (Cella M. et al., J. of Exp. Med. 184:747-52 (1996); Ferlin W. G. et al., Europ. J. of Immunol. 28:525-31 (1998); Armant M. et al., Europ. J. of Immunol. 26:1430-4 (1996); Koch F. et al., J. of Exp. Med. 184:741-6 (1996); Seguin R. and L. H. Kasper, J. of Infect. Diseases 179:467-74 (1999); Chaussabel D. et al., Infection & Immunity 67:1929-34 (1999)), IL-15 (Kuniyoshi J. S. et al., Cellular Immunol. 193:48-58 (1999)) and chemokines (MIP1α, MIP1β, RANTES, and others) (McDyer J. F. et al., J. of Immunol. 162:3711-7 (1999); Schaniel C. et al., J. of Exp. Med. 188:451-63 (1998); Altenburg A. et al., J. of Immunol. 162:4140-7 (1999); Deckers J. G. et al., J. of the Am. Society of Nephrology 9:1187-93 (1998)), increased expression of MHC class I and II (Santos-Argumedo L. et al., Cellular Immunol. 156:272-85 (1994)), and increased expression of adhesion molecules (e.g., ICAM) (Lee H. H. et al., Proc. Natl. Acad. Sci. USA. 96:1421-6 (1999); Grousson J. et al., Archives of Dermatol. Res. 290:325-30 (1998); Katada Y. et al., Europ. J. of Immunol. 26:192-200 (1996); Mayumi M. et al., J. of Allergy & Clin. Immunol. 96:1136-44 (1995); Flores-Romo L. et al., Immunol. 79:445-51 (1993)) and costimulatory molecules (e.g., B7) (Roy M. et al., Europ. J. of Immunol. 25:596-603 (1995); Jones K. W. and C. J. Hackett, Cellular Immunol. 174:42-53 (1996); Caux C. et al., Journal of Exp. Med. 180:1263-72 (1994); Kiener P. A. et al., J. of Immunol. 155:4917-25 (1995)). Cytokines induced by CD40 engagement enhance T cell survival and activation.
In addition to enhancement of cellular and immune function, the effects of CD40 activation include: cell recruitment and differentiation by chemokines and cytokines; activation of monocytes; increased cytolytic activity of cytolytic T lymphocyte (CTL) and natural killer (NK) cells; induction of apoptosis in CD40 positive tumors; enhancement of immunogenicity of CD40 positive tumors; and tumor-specific antibody production. The role of CD40 activation in cell-mediated immune responses is also well established, and it is reviewed in: Grewal et al., Ann. Rev. of Immunol. 16:111-35 (1998); Mackey et al., J. of Leukocyte Biol. 63:418-28 (1998); and Noelle R. J., Agents & Actions-Suppl. 49:17-22 (1998).
Studies using a cross-priming model system showed that CD40 activation of APCs can replace helper T cell requirement for the generation of cytolytic T lymphocyte (CTL). (Bennett et al., Nature 393:478-480 (1998).) Evidence from CD40L deficient mice indicates a clear requirement for CD40 signaling in helper T cell priming. (Grewal I. S. et al., Science 273:1864-7 (1996); Grewal I. S. et al., Nature 378:617-20 (1995).) CD40 activation converts otherwise tolerogenic, antigen bearing B cells into competent APCs. (Buhlmann J. E. et al., Immunity 2:645-53 (1995).) CD40 activation induces maturation and differentiation of cord blood progenitors into dendritic cells. (Flores-Romo L. et al., J. of Exp. Med. 185:341-9 (1997); Mackey M. F. et al., J. of Immunol. 161:2094-8 (1998).) CD40 activation also induces differentiation of monocytes into functional dendritic cells. (Brossart P. et al., Blood 92:4238-47 (1998).) Further, CD40 activation enhances cytolytic activity of NK cells through APC-CD40 induced cytokines. (Carbone E. et al., J. of Exp. Med. 185:2053-60 (1997); Martin-Fontecha A. et al., J. of Immunol. 162:5910-6 (1999).) These observations indicate that CD40 plays an essential role in the initiation and enhancement of immune responses by inducing maturation of APCs, secretion of helper cytokines, upregulation of costimulatory molecules, and enhancement of effector functions.
The critical role of CD40 signaling in the initiation and maturation of humoral and cytotoxic immune responses makes this system an ideal target for immune enhancement. Such enhancement can be particularly important for mounting effective immune responses to tumor antigens, which are generally presented to the immune system through cross-priming of activated APCs. (Huang A. Y. et al., Ciba Foundation Symp. 187:229-44 (1994); Toes R. E. M. et al., Seminars in Immunol. 10:443-8 (1998); Albert M. L. et al., Nature 392:86-9 (1998); Bennett S. R. et al., J. of Exp. Med. 186:65-70 (1997).)
Several groups have demonstrated the effectiveness of CD40 activation for antitumor responses in vitro and in vivo. (Toes R. E. M. et al., Seminars in Immunol. 10:443-8 (1998).) Two groups, using lung metastatic model of renal cell carcinoma and subcutaneous tumors by virally transformed cells, have independently demonstrated that CD40 activation can reverse tolerance to tumor-specific antigens, resulting in efficient antitumor priming of T cells. (Sotomayor E. M. et al., Nature Medicine 5:780-787 (1999); Diehl L. et al., Nature Medicine 5:774-9 (1999).) Antitumor activity in the absence of immune cells was also reported by CD40L and anti-CD40 antibody treatment in a human breast cancer line model in SCID mice. (Hirano A. et al., Blood 93:2999-3007 (1999).) CD40 activation by anti-CD40 antibody was recently shown to eradicate CD40+ and CD40-lymphoma in mouse models. (French R. R. et al., Nature Medicine 5:548-53 (1999).) Furthermore, previous studies by Glennie and co-workers conclude that signaling activity by anti-CD40 antibodies is more effective for inducing in vivo tumor clearance than other anti-surface marker antibodies capable of recruiting effectors. (Tutt A. L. et al., J. of Immunol. 161:3176-85 (1998).) Consistent with these observations, when anti-CD40 antibodies were tested for activity against CD40+ tumor cells in vivo, most but not all of the tumoricidal activity was associated with CD40 signaling rather than ADCC. (Funakoshi S. et al., J. of Immunotherapy with Emphasis on Tumor Immunol. 19:93-101 (1996).) In another study, bone marrow dendritic cells were treated ex vivo with a variety of agents, and tested for in vivo antitumor activity. These studies demonstrated that CD40L stimulated DCs were the most mature and most effective cells that mounting an antitumor response.
The essential role of CD40 in antitumor immunity has also been demonstrated by comparing responses of wild-type and CD40−/− mice to tumor vaccines. These studies show that CD40−/− mice are incapable of achieving the tumor immunity observed in normal mice. (Mackey M. F. et al., Cancer Research 57:2569-74 (1997).) In another study, splenocytes from tumor bearing mice were stimulated with tumor cells and treated with activating anti-CD40 antibodies ex vivo, and were shown to have enhanced tumor specific CTL activity. (Donepudi M. et al., Cancer Immunol. Immunother. 48:153-164 (1999).) These studies demonstrate that CD40 occupies a critical position in antitumor immunity, in both CD40 positive and negative tumors. Since CD40 is expressed in lymphomas, leukemias, multiple myeloma, a majority of carcinomas of nasopharynx, bladder, ovary, and liver, and some breast and colorectal cancers, activation of CD40 can have a broad range of clinical applications.
Anti-CD40 activating monoclonal antibodies can contribute to tumor eradication via several important mechanisms. Foremost among these is activation of host dendritic cells for enhanced tumor antigen processing and presentation, as well as enhanced antigen presentation or immunogenicity of CD40 positive tumor cells themselves, leading to activation of tumor specific CD4+ and CD8+ lymphocytes. Additional antitumor activity can be mediated by other immune-enhancing effects of CD40 signaling (production of chemokines and cytokines, recruitment and activation monocytes, and enhanced CTL and NK cytolytic activity), as well as direct killing of CD40+ tumors by induction of apoptosis or by stimulating a humoral response leading to ADCC. Apoptotic and dying tumor cells can also become an important source of tumor-specific antigens that are processed and presented by CD40 activated APCs. Accordingly, there is a critical need for therapeutic, clinically relevant anti-CD40 agonist antibodies.